The objective of this proposal is to characterize a fundamental calcium signaling mechanism that regulates long lasting intracellular adaptive responses to light stimulation and, when abnormal, causes photoreceptor degeneration. We will determine how depletion of intracellular calcium stores triggers calcium influx through store-operated calcium channels in the plasma membrane of photoreceptors. The molecular isoforms of store-operated channels will be identified and their role in regulating the amplitude and frequency of spontaneous synaptic responses and light-evoked responses, determined. The mechanisms, through which pathological calcium influx through store-operated channels kills photoreceptors will be characterized in mouse model systems for inherited retinal degeneration and in mice lacking store-operated (TRPC) channel isoforms. Two Specific Aims are proposed: Specific Aim 1: To characterize the roles of store-operated calcium channel entry in photoreceptor calcium homeostasis and neurotransmission Specific Aim 2: To identify expression of transient receptor potential channels (TRPCs) in the retina, and to determine their role in store-operated calcium entry and visual signaling The experiments in isolated cells, retinal slices and TRPC knockout mice will combine high-resolution calcium imaging techniques with patch clamp electrophysiology, immunocytochemistry, RT-PCR, gene chip and RNAi technology. Depletion of calcium stores and dysfunctional calcium influx kill photoreceptors. Thus, the proposed studies will help us understand how photoreceptors adjust their calcium levels to maintain optimal signaling range and will also help us identify potential candidate cellular targets for therapeutic interventions during retinal disease.